Polarity determination that shows whether a sentence is an affirmative expression or a negative expression is used for reputation analysis and the like. Polarity determination includes a method whereby a dictionary in which affirmative words and negative words are recorded in advance is kept, the affirmative words and negative words in a sentence is counted based on the dictionary, and the polarity of the majority is determined to be the polarity of the sentence. However, because sentences may include negations, retractions and the like, a correct determination may not be made simply by counting the words representing affirmative expressions and words representing negative expressions. One polarity determination method statistically analyzes the tendency of appearance of a phrase consisting of not more than n continuous words in affirmative sentences and negative sentences. In this case, if the positions of a negative word and the object of negation in a sentence are close (not more than n), there is a possibility of correct determination. However, as n increases, the number of combinations may become huge. Accordingly, realistically n=3, more or less, is used.
Another polarity determination method employs a dependency structure rather than the positional proximity relationship in the sentence. Non Patent Literature 1 describes a polarity determination method using the dependency structure.